There is a need for an accurate estimation of absolute dose within planes, by way of example in a phantom, based on an input of Mega-voltage (MV) EPID images. As described in U.S. Pat. Nos. 6,345,114 for Method and Apparatus for Calibration of Radiation Therapy Equipment and Verification of Radiation Therapy, and 6,839,404 for System and Method for Positioning an Electric Portal Imaging Device, EPIDs is well known. Advantages of the EPID include'online convenience, data resolution (small pixels), and data density. However, MV EPIDs are not dosimeters, as the interactions of photons leading to an EPID image are notably different than the interactions in water or tissue that lead to radiation dose. It is desirable to maintain the industry standard of measured dose-to-calculated dose to perform IMRT QA analysis. Therefore comparison of anything other than dose, such as comparing a measured image to a predicted image is an undesirable shift from comparing a measured dose plane to a calculated dose plane. Additionally, percent differences, distance to agreement (DTA), and gamma criteria used in IMRT QA are based on a dose-in-tissue/water rationale.
There is also a need for an independent and reliably robust analysis. Any QA solution that is built into a radiation delivery system is a “self check,” which results in a fundamental conflict of interest due to the lack of a 3rd party independence. By way of example, exposing potential errors via independent and rigorous QA is a high ranking goal of medical physics. Relying completely on one system for planning, delivery, and QA reduces the likelihood of catching errors due to shared components, internal biases, and conflicting objectives.
Typical EPID images have pixel values that do not have dose equivalent units, for example are not centi-Gray (cGy). As a result using raw EPID images generates comparisons that are quantitative but not dosimetric. In such a case, use of standard intensity modulated radiography quality assurance (IMRT QA) analysis tools (DTA and gamma especially) is questionable unless acceptance criteria are re-established for non-dosimetric images. Additionally, the EPID image is typically acquired in a different geometry than is typical for IMRT QA. In other words, it is acquired at a different source-to-detector distance and with different build-up characteristics.
By way of further example, EPID detectors exhibit a different response with respect to energy spectra and scatter radiation than do dosimeters providing dose in tissue/water. Further, EPID images have a point spread response that is different than a dose kernel superposition of dose at depth in tissue/water. Yet further, EPID images exhibit off-axis/wide field variations in response.
There is a need for EPID-to-Dose conversions that allow IMRT QA to remain dose-based. There is also a need to estimate an absolute dose delivered in standard IMRT QA conditions including factors such as tissue equivalent buildup, source-to-detector distance, dose to tissue, and the like.